The present invention relates to an integrated circuit structure with complementary MOS transistors, comprising a semiconductor substrate in which are formed a first type of MOS transistor of a given type of conduction and at least one pocket doped opposite that of the substrate, in which is formed a second type of MOS transistor, of a type of conduction opposite the first.
In a complex circuit it is frequently necessary to arrange, on one and the same integrated circuit, MOS transistors of both types, and other elements which can serve as stable sources of current or high resistors. However, the present state of the art does not make is possible satisfactorily to meet this need. In fact, these additional elements are difficult to obtain by means of known structures. In order to obtain sources of current, for example, one must have recourse to circuit artifices which are not always satisfactory. Resistors formed of pocket diffusions have too high a doping and therefore too low a resistivity for certain applications.
Strictly speaking, it may be possible to supplement known technology by adding additional stages and masks to improve the characteristics of these elements. However, in practice the addition of one more stage or mask to the overall fabrication process results in a very substantial increase in the cost of the integrated circuit and more severe requirements in its fabrication if good quality is desired.
Accordingly, the object of the invention is to provide a complementary MOS transistor integrated circuit comprising additional circuit elements or electronic components which can serve as stable sources of current or high resistors without resulting in any substantial complication of the fabrication technology.
The MOS transistor integrated circuit structure constructed in accordance with the invention is characterized by the fact that it comprises an electronic component having a region of doping the concentration of which is intermediately between that of the substrate and of the pockets formed in the substrate. This region is formed by the vicinity of two pocket edges, the edges being separated by a distance which is substantially not greater than twice the length of the lateral diffusion of the doping of the pockets. The doping of this region is determined by the lateral diffusion of the pockets.
The invention therefore satisfies the above-mentioned need by providing a circuit whose construction does not involve any intolerable modification in existing technology, but simply certain details in the layout of the photolighographic masks. These pecularities consist essentially, as will be described, in providing either a narrow masking band in a pocket or a relatively narrow space between two pocket regions.
As background for the invention, one should consult the technology described in U.S. Pat. No. 3,646,665 to Kim or Swiss Pat. No. 542,518 which discloses an integrated circuit structure having complementary MOS transistors with a gate of silicon and doped oxides.
The obvious advantage provided by the present invention over the structure disclosed in this patent resides in the additional circuit components produced by means of the present invention.
These components may be resistors, with the advantage of a higher resistivity than that of conventional resistive pockets. These additional components may also be transistors which have a lower threshold voltage than that of the other transistors of the circuit which are made in known manner. One can even provide a threshold voltage such that a current flows with zero gate voltage, the component then constituting a very practical source of current. The threshold voltage may be selected between various values by selecting an appropriate width for the space between the two pocket edges. For an n-channel MOS transistor, for instance, if the width is very small the properties approach those of known transistor elements. If this width is on the order of magnitude of the junction depth, the intermediate doping region will have a very slightly doped zone and will have a lowered threshold voltage. However, this width can barely exceed twice the length of the lateral diffusion, otherwise the lateral diffusions would be disjointed and the component would no longer operate.
An integrated circuit is known having complementary MOS transistors and a third type of component. However, since the additional component in this case is a high-voltage transistor, problems are involved which are different from those addressed by the present invention. The starting technology is of the type described in U.S. Pat. No. 3,646,665 cited above and the methods of obtaining the high voltage transistor are described in the article by R. A. Blanchard et al entitled "High Voltage Simultaneous Diffusion Silicon-Gate CMOS", IEEE J. of Solid State Circuits, Vol. SC-9, No. 3, June 1974, pp. 103-110 (hereinafter Ref. 2).
Nothing in this article suggests the concept which forms the basis of the present invention, and the result obtained by the process disclosed therein is entirely different from that obtained by the present invention.
An integrated circuit structure is also known which involves the idea of using lateral diffusion. However, this structure is directed to a very different purpose from that of the present invention, namely, to obtain a very short channel, without special requirements for the mask. This purpose and the manner of carrying it out is described in the article by M. D. Poscha et al "Threshold Voltage Controllability in Double-Diffused MOST", IEEE Trans. on Electron Devices, Vol. ED-21, No. 12, Dec. 1974 (hereinafter Ref. 3).
In accordance with this article, a so-called double diffused MOS transistor comprises a very short channel obtained by lateral diffusion. The length of the channel is substantially the difference in lateral penetration of two successive diffusions of opposite doping agents diffused from the same region. The structure obtained is very different from that which is proposed in the present invention.
The latter article refers in particular to the problem which consists in controlling the threshold voltage of the fabricated transistor. But the purpose of the double-diffused MOST having a short channel in no way relates to a device having a reduced or zero threshold voltage; rather, the purpose is to obtain a transistor capable of good high frequency performance, particularly as a result of a short channel capable of conducting a high current with relatively low ohmic drop.
Moreover, with respect to the threshold voltage, it will be appreciated that the present invention makes it possible to definitely control this parameter, to the extend that transistors of different threshold values can be created as desired on the same circuit, by simply designing the masks accordingly.
The transistor described in the latter article raises certain problems as soon as one attempts to incorporate it, as component, in a integrated circuit structure. One of the problems is the insulation of the drain.
A technique which has made it possible to overcome this difficulty is described in the article by T. Masuhara and R. S. Muller "Complementary D-MOS Process for LSI" IEEE J. of Solid State Circuits, Vol. SC-11, No. 4, August 1976 (hereinafter Ref. 4).
The purpose of the technique described in this article is to create a new variety of integrated circuit with complementary MOS transistors. The CMOS's are extremely interesting for logical circuits of high density and low static current consumption. The idea presented consists in replacing, in the CMOS circuit, the transistors of one type, for instance the n-channel MOS's if the substrate is of n type, by a double-diffusion MOS transistor (DMOS). The DMOS described in this article is different from that of the Poscha et al article mentioned above (Ref. 3). The effects obtained consist primarily of a decrease in the number of masks necessary and in a saving in diffusion time and pocket area.
The present invention has importance in its own right, in view of the fact that it satisfies other requirements and produces other effects not discussed in this article. In particular, one of the advantages of the present invention is to add to a conventional CMOS structure components which assume functions as resistors or sources of current which previously could not be satisfactorily realized. This is entirely different from what is described in this article.
The invention will be better understood by means of the description given below with reference to the accompanying drawings, which illustrate, by way of example, several embodiments of the invention.